I would say a 75 would be fine. Just expect to lose the tiger barbs. Good luck! Also don't get too attached to the firemouths either, they will be gone in a matter of years.
I was told to expect to lose barbs but also every1 says every fish are diff and my convicts hav had 2 batches of fry and ther are still eggs in the tabk and everyonther fish is fune they even swim near the fry/ eggs and dont bother with the convicts so im not expecting to lose my barbs